The invention relates to a method and a system for introducing particulate solids into a substrate.
In the following, especially papers, in particular carrier papers or decorative papers, but also boards and panels of plastic material, wood or wood-based material, which are used on the ceiling, wall or floor, will be referred to as substrates.
Applying particulate solids is interesting in particular, when thin layers of particulate solids are to be applied on a substrate. The particulate solids are relatively heavy and hard. They have a specific weight of more than 2 g/cm3, often more than 3 g/cm3. The Mohs-hardness amounts to 8-10. The typical case of application of the present invention is the application of corundum, silicates or other particulate solids on a substrate in order to improve its surface properties. When corundum is applied, for example, on synthetic resin layers and surface coatings thereof, their abrasion resistance is improved. The essential precondition is that the particulate solids do not form the surface of the substrate. They must be embedded in a layer near the surface to achieve improved abrasion resistance, for example. Silicates dispersed in synthetic resin layers improve, for example, the scratch resistance of surface coatings.
Various technical approaches are known for the application of such particulate solids. A group of approaches aims at binding the particulate solids in liquids from the start, to then roll them on, cast them on or to spread them. In particular, WO 00/44984, DE 196 04 907 or DE 195 08 797 describe a dispersion containing particulate solids. This dispersion is applied by a flushing nozzle from below onto each substrate to be coated. Further it is suggested that additives be added to such dispersions to improve the handling of such particulate solids. The additives can be fibers and/or spherical bodies (glass spheres). It is disadvantageous that the manufacture and processing of the dispersion is very troublesome, as it has to be avoided, that the particulate solids settle. Even after short interruptions of production, the application apparatus must be fully cleaned, since the dispersion will otherwise block lines and nozzles. Moreover, the application amount varies strongly.
As an alternative, WO 2005/042644 suggests sprinkling such particulate solids onto the substrate. Particulate solids are sprinkled on the substrate via a roller arrangement, below which the substrate is passed. This arrangement is simple in its mechanic setup and operation, but the uniformity and precision of the application is not satisfactory and mechanical wear is very high. Moreover, in a second step, fibers must be applied on the surface of the substrate in a very troublesome manner and selectively oriented in order to ensure the embedding of the particulate solids.
All approaches must face the problem that the particulate solids, such as corundum, silicates or other particles are mostly extremely abrasive. The methods known from the state of the art therefore attempt to make the contact between the particulate solids and the corresponding conveying apparatus as delicate as possible to minimize the abrasion on the conveying apparatus. Embedding the particulate solids in synthetic resin, which is extremely troublesome in practice, effectively envelopes the particles. Sprinkling via a roller arrangement avoids interfering friction as far as possible.